The Government has been accused of denying the "rights" of Chinese artist Ai Weiwei after he was denied a six month visa to visit the UK.

The artist, famous for designing Beijing's Olympic stadium and filling Tate Modern's Turbine Hall with ceramic sunflower seeds, is travelling to London for a landmark exhibition of his work after Chinese authorities granted him a passport.

He posted a picture on his Instagram account of his passport with a UK visa for 20 days from September 9, but also included a letter from the British embassy in Beijing saying he had been refused a six-month visa.

The letter said it had been refused because he said he had never had a criminal conviction even though "it is a matter of public record that you have previously received a criminal conviction in China".

Weiwei, an o utspoken critic of China's human rights record, w as arrested and imprisoned in April 2011 as he boarded a Beijing flight bound for Hong Kong, prompting a global campaign leading to his release.

A statement on his site said he had "never been charged or convicted of a crime".

It added: " This decision is a denial of Ai Weiwei's rights as an ordinary citizen."

Tim Marlow, the artistic director of the Royal Academy in central London which is hosting the exhibition, said the artist was "bemused and irate".

He told BBC Radio 4's World at One: " The letter said that entry was being granted for the period he requested, from September 9 to 29, but the six-month visa was denied 'because it is a matter of public record that you have previously received a criminal conviction in China' and he hadn't declared it.

"It is absolutely not a matter of public record. He has never received a criminal conviction in China. The notion you have to prove your innocence is an odd one, but he would have done if he had been given a week or two's notice to do that.

"Why would someone in the British Embassy in Beijing not understand that what happened to Ai Weiwei was widely reported in the world's media, but nobody reported a criminal conviction? He was held in detention for 81 days, but there was never a criminal charge or conviction made."